Could someone please explain how teachers' unions protecting "bad teachers" from being fired is in the best interests of the teachers unions? They get accused

Could someone please explain how teachers' unions protecting "bad teachers" from being fired is in the best interests of the teachers unions? They get accused of that a lot, but I can't see how that would work for them. If my coworkers were making me look bad, I sure wouldn't want to associate myself with them. It seems to me it would weaken the credibility of the union and thus take away the power the union has to back me up.